You are discussing two separate things in the same point. Whether there is an increment or not, there should be a notice period attached to any separation depending on the company's policy.
Contractor Employment and Notice Period
Secondly, if you are working as a contractor, your principal employer would provide an offer letter that you need to confirm with your employer that you agree to the said increment. Even in this scenario, there would be a notice period that should be agreed upon with your employer. If you have agreed to the increment and then resigned, the effective amount should be paid for the period after the said agreement.
Hope this clarifies.
Regards,
Vivek